This invention relates generally to navigation of air vehicles, and more specifically, to methods and systems for enhancing accuracy of terrain aided navigation systems.
Precision terrain aided navigation (PTAN) correlates interferometric Doppler radar ground return data with a digital elevation map (DEM), resulting in position updates that are provided to a navigation system. Typical navigation systems incorporate at least two sources of navigation data to provide a total position solution. For example, position data from an inertial navigation system (INS), can be combined with position data from PTAN to provide a total position solution. The radar updates from PTAN are utilized to subtract out drift errors that occur within inertial sensor systems like the INS.
Accuracy of the PTAN system relies somewhat on the type of terrain features over which the aircraft is flying. For example, PTAN provides a very low accuracy update over featureless terrain such as water, or flat desert since it is difficult to correlate the featureless terrain with the data stored in the DEM. Conversely, high levels of accuracy are provided over mountainous terrain by a PTAN system because of the ability to correlate the rapidly changing terrain features with the DEM data. Urban areas are similar to mountainous terrain with respect to accuracy of the PTAN system, specifically, the existence of terrain elevation changes as the aircraft moves along its flight path allows for easy correlation with data stored in the DEM.